Polyamides are synthetic polymers widely used for the manufacture of diverse shaped articles, including moulded and injected parts, which are often proposed for the high end electrical, electronic, and automotive industry.
In these fields of use, the moulded polyamide article during its normal useful lifetime is in contact with a heat source which frequently attains and/or which attains for a longer period temperatures largely exceeding 100° C. The heat source may be a heat producing device or a heated device or may be the surrounding environment wherein the moulded article is placed. Examples of heated devices or heat generating devices are engines, or elements thereof, and electronic devices such as semiconductors. For the automotive segment high-temperature-use application are regularly found in so-called under-the-hood or under-the-bonnet applications, herein referred to as high temperature automotive applications. Therefore, the invention in particular relates to polyamide suitable for the manufacture of moulded articles for use in the electro-, electronic, and automotive industry.
Moulded articles for the electrical, electronic and automotive industry and moulding compositions based on polyamides generally have to comply with a complex property profile, including, for the compositions as moulded, good dimensional stability, high heat distortion temperature (HDT) and good mechanical properties, such as a high tensile strength, tensile modulus and fatigue. Polyamide materials generally tend to show a decrease in mechanical properties due to thermal degradation of the polymer. This effect is called heat ageing. This effect can occur to an undesirable extent. In particular with polyamides as the thermoplastic polymer, the deteriorating effect of exposure to high temperatures can be very dramatic.
In attempts to improve heat aging characteristics, it has been conventional practice to add heat stabilizers to polyamide compositions. The function of a heat stabilizer is to better retain the properties of the composition upon exposure of the moulded article to elevated temperature. When using a heat stabilizer, the useful lifetime of the moulded material can be extended significantly, depending on the type of material, use conditions and type and amount of heat stabilizer. Examples of heat stabilizers typically used in polyamides are organic stabilizers, like phenolic antioxidants and aromatic amines, and copper, either in the form of a copper salt in combination with potassium iodide or potassium bromide, or in the form of elementary copper, and metal powders, in particular iron powders.
Existing technologies, while leading to improvements of long-term heat aging resistance, are nevertheless insufficient for more demanding applications, involving exposure to higher temperatures; in many applications, retention of mechanical properties after long-term exposure to temperatures as high as 160° C., or even 180-200° C. and higher becomes a basic requisite. The number of specialty applications, requiring compositions with improved heat ageing properties is also increasing.
The aim of the invention is therefore to provide polyamide compositions, which have better heat ageing properties than the known compositions, thereby providing for the possibility to make moulded articles that can be used at higher continuous use temperatures than the moulded articles prepared with the known compositions and which possess outstanding impact strength.
Within this scenario, U.S. Pat. No. 4,945,129 (ALLIED SIGNAL) Jul. 31, 1990 discloses a polyamide composition comprising (i) an amine-terminated polyamide, which is preferably of polycaprolactam-type, and which can comprise branching materials such as Jeffamine products (i.e. propylene oxide triamines), or tri- and tetra-functional ethylene amines; (ii) another additional polyamide and (iii) an olefin reactive copolymer. The composition can further comprise stabilizers and inhibitors of oxidative, thermal and UV light degradation, with combinations of Group I metal halides and cuprous halides being mentioned. Such compositions are shown as possessing improved impact strength, in particular at low temperature.
Further, WO 2013/004531 (DSM IP ASSETS BV) Jan. 10, 2013 discloses a polyamide composition comprising:                a pre-polymer Y, obtained from polycondensation of polyfunctional monomer and mixture of AA/BB monomers and AB monomers, having a molecular weight of 600-3500 g/mol, said pre-polymer having preferably unbalanced end groups, and in particular an excess of amine end groups;        a first linear pre-polyamide X1 predominantly consisting of AA-BB repeating units; and        a second linear pre-polyamide X2 predominantly consisting of AB repeating units. The composition can further comprise usual additives, e.g. heat stabilizers and antioxidants. The above composition is mixed in the molten state to provide by reactive extrusion a high molecular weight branched polyamide, in which the difference between concentration of amine and carboxylic acid end groups (AEG−CEG) ranges generally of from 0 to 35 meq/kg (i.e. providing for a slight excess of amine end groups).        
Similarly, WO 2013/004548 (DSM IP ASSETS BV) Jan. 10, 2013 discloses a polyamide composition comprising:                a pre-polymer Y, obtained from polycondensation of polyfunctional monomer and mixture of AA/BB monomers, having a molecular weight of 600-3500 g/mol, said pre-polymer having preferably unbalanced end groups, and in particular an excess of amine end groups;        a linear pre-polyamide X consisting of AA-BB repeating units. The composition can further comprise usual additives, e.g. heat stabilizers and antioxidants. The above composition is mixed in the molten state to provide by reactive extrusion a high molecular weight branched polyamide, in which the difference between concentration of amine and carboxylic acid end groups (AEG−CEG) ranges generally of from 0 to 35 meq/kg (i.e. providing for a slight excess of amine end groups).        
The Applicant has now found that by the incorporation in compounds based on polyamides of a combination of a well-defined amount of a copper-containing stabilizer and a well defined amount of a particular branched polyamide comprising a significant amount of amine end groups in excess over carboxylic acid end group is effective in delivering outstanding synergetic heat aging stability effect, in particular delivering outstanding retention of mechanical properties even after long term exposure to temperatures as high as 210° C., while simultaneously providing toughening effect.